Precision Engineering 27 (2003) 258–264

Development of RF magnetron sputtering method tofabricate PZT thin film actuator

Kazuyoshi Tsuchiya∗, Toshiaki Kitagawa, Eiji NakamachiOsaka Institute of Technology, Bio Venture Center, 5-16-1 Oohiya Asahi-Ku, Osaka 535-8585, Japan

Received 21 August 2002; received in revised form 2 December 2002; accepted 11 December 2002

Abstract

PZT piezoelectric very thin films suitable for a microactuator have been deposited onto Invar alloy substrate using a high-temperatureRF magnetron sputtering technique. PZT thin films must be deposited onto conductive substrate for a monomorph or a bimorph actuator.The chemical composition and the crystalline structure of these films were measured by ESCA and XRD, respectively. The chemicalcomposition of PZT deposited stoichiometrically was almost the same as commercially-produced bulk PZT. Crystal planes (1 1 0) and(1 1 1) of PZT perovskite structure were observed in XRD analysis. When the substrate was heated to above 600◦C, SEM revealed only avery small number of pinholes on the surface. A thin (500 nm) film actuator has been characterized by measuring the piezoelectric propertyusing a Laser Doppler Vibrograph. It was confirmed that the piezoelectric property has a linear relationship with the grain size, which alsoincreased with the substrate temperature. The piezoelectric property of deposited PZT thin films showed a good agreement with a quotedvalue of bulk PZT, when the substrates were heated to 600◦C.© 2003 Elsevier Science Inc. All rights reserved.

Keywords:PZT; RF magnetron sputtering; Piezoelectric property; Grain size; Substrate temperature

1. Introduction

PZT piezoelectric thin films can be promising candidatesfor microactuators or microsensors in micro electro mechan-ical systems (MEMS)[1] because of their high power output,and the generation of large displacements without the needfor gears or motors. Conventional PZT production techniquessuch as the sol–gel method[2], the hydrothermal method[3] and the sputtering method can produce thin films for mi-crosensors or microactuators in sub-micron or micrometerorder thickness. However, PZT thin films produced by theseconventional methods cannot easily replicate the piezoelec-tric properties of bulk PZT produced by the sintering method.

Sputtering deposition methods require the heat treatmentfor crystallization of perovskite structures.Fig. 1 shows aschematic diagram of our high-temperature RF magnetronsputtering equipment, improved by the O-Naru Tech Cor-poration (N-SPCVD-11). The temperature of the substratecan be raised to 900◦C, whilst the thin film is depositedunder high vacuum. It is not therefore necessary to annealat high temperatures following the deposition. The target ofPb1.2(Zr0.52, Ti0.48)O3 that we used had a lead composition

∗ Corresponding author. Fax:+81-6-6957-2134.E-mail address:esrmr@mmsun1.med.oit.ac.jp (K. Tsuchiya).

20% greater than the stoichiometric target because, at hightemperatures, lead particles evaporate easily from the de-posited films. Deposition conditions such as (1) argon andoxygen pressure, (2) input power, and (3) substrate tempera-ture were investigated as very thin PZT films were fabricatedonto Invar alloy substrates. Finally, the piezoelectric propertyof the PZT thin film actuator is measured and compared withthat of commercially-produced bulk PZT.

2. Sputtering conditions for PZT thin film deposition

2.1. Argon pressure and input power for PZT deposition

According to the conventional process for thin film depo-sition, (1) argon pressure and (2) input power were investi-gated by an assessment of the deposition rate and the surfaceroughness. Generally, the sputtering pressure has a significanteffect on the deposition rate. At first, argon gas was selectedto find an optimum sputtering pressure in order to maximizethe deposition rate at an input power of 100 W. As shown inFig. 2, the maximum deposition rate of 0.2 nm/s occurs whenthe argon pressure is 2 Pa. At higher pressures, there is a pro-gressive decrease in the rate of deposition because sputteredatoms are scattered by residual gas particles. Secondly, we

0141-6359/03/$ – see front matter © 2003 Elsevier Science Inc. All rights reserved.doi:10.1016/S0141-6359(03)00006-0

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Fig. 1. Schematic diagram of RF magnetron sputter equipment.

Fig. 2. Relationship between film thickness and argon pressure.

varied the input power to keep the surface roughness, Ra, be-low 5 nm whilst maintaining a high deposition rate. AsFig. 3indicates, the selected value of 100 W leads to a depositionrate of 0.2 nm/s and surface roughness of Ra= 4 nm. It wasfound that the smoother the surface, the greater the piezo-electric function.

2.2. Oxygen pressure and flow rate

Generally, in order to fabricate oxidized thin films us-ing sputtering deposition methods, ceramic targets areused. However, deposition rates are very slow and the

Fig. 3. Surface roughness changing as a function of input power.

Fig. 4. Oxygen pressure as a function of oxygen flow rate.

thin film composition cannot be stoichiometric because ofre-sputtering. According to Hata, there is a nonlinear rela-tionship between the deposition rate and the gas pressurefor reactive sputtering. The speedy part is called the metal-lic mode and the slow part is called the oxidized mode[4,5]. According to Kinbara et al., there is also a nonlinearrelationship between the oxygen flow rate and the oxygenpressure[6]. Furthermore, when the deposition rate is fastin the metallic mode, the target surface will be oxidized.Therefore, at this time, the oxygen pressure is kept low inthe sputtering chamber. However, when the oxidization ofthe target surface is complete, the oxygen pressure increasessuddenly, and residual oxygen particles will be neutralized inthe plasma. Neutralized oxygen particles are accelerated andbombard the substrate or target, causing a dramatic increasein the deposition rate.

In order to investigate the relationship between the oxy-gen pressure and oxygen flow, the critical phase betweenthe metallic mode and the oxidized mode was investigatedusing Ti target. Initially, the Ti surface was sufficientlywell-oxidized with an oxygen flow rate of 9 sccm, at 2 Pa.At 30-min intervals, the oxygen flow rate was decreased by1 sccm and the oxygen pressure was measured. The relation-ship is shown inFig. 4. The phase from oxidized mode tometallic mode appeared between 6 and 5 sccm. Consequently,in this experiment, we used values of 5 sccm and 0.5 Pa forthe oxygen flow rate and oxygen pressure, respectively.

3. Substrate for PZT deposition

For the sputtering deposition, an appropriate substrate mustbe chosen. When PZT thin films are deposited, the substratesheat up as PZT grains are grown on the substrate. In orderto produce a PZT thin film with good piezoelectric prop-erty, there should be no cracks either on the surface or withinthe substrate. Therefore, the lattice constant for the substratemust be close to the value for the PZT. Moreover, the substrate

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Fig. 5. AFM images of deposited PZT on: (a) Si substrate and (b) Invar alloy substrate.

Table 1Electric properties for PZT and Invar alloy

Composition Linear expansioncoefficient (◦C−1)

Resistivity

PZT Pb(Zr0.52, Ti0.48)O3 5.0–10× 10−6 1010–1011 �mInvar Fe58/Ni42 1.7–2.0× 10−6 0.75–0.85��m

Table 2Sputtering conditions and targets

Sample Target Condition

1 Pb(Zr0.52, Ti0.48)O3 Ar 2 Pa for 2 h2 Pb1.2(Zr0.52, Ti0.48)O3 Ar:O2 = 3:1, over 470◦C

heat up for 2 h3 Pb1.2(Zr0.52, Ti0.48)O3 Ar:O2 = 3:1, over 600◦C

heat up for 2 h4 Bulk PZT (Fuji Ceramics)

must be conductive in order to control the actuator-appliedvoltage.Table 1shows electric properties for PZT and theInvar (iron/nickel) substrate. The thermal expansion coeffi-cients for Invar and PZT are very close, and the resistivity issmall enough to be used as an electrode.

In conventional processes to produce PZT micro sensorsor actuators, Si substrates are very often used because the re-sistivity and lattice constant are similar to PZT.Fig. 5showsAFM images of deposited PZT thin film surface on (a) a Sisubstrate and (b) an Invar alloy substrate, under the samesputtering conditions. The grains of PZT are deposited moreuniformly onto the Si substrate than the Invar substrate. How-ever, the grain sizes deposited onto the Si and Invar alloysubstrates were almost identical (average grain sizes were230 and 250 nm, respectively). In this experiment, Invar alloysubstrates were used for PZT deposition (Table 2).

4. Chemical composition and crystalline structure ofdeposited PZT thin films

The chemical composition of the PZT piezoelectric actua-tor has a significant effect on its piezoelectric property. Elec-tron spectroscopy for chemical analysis (ESCA) was used in

Fig. 6. Chemical composition of PZT thin films deposited at various condi-tions.

order to measure the chemical composition of lead, zirco-nium, titanium and oxygen within the PZT.Fig. 6shows thechemical composition of the PZT thin films under variousdeposition conditions.

Sample 1 was deposited for 2 h using a Pb(Zr0.52, Ti0.48)O3target. The argon pressure was 2 Pa. Samples 2 and 3 weredeposited using a Pb1.2(Zr0.52, Ti0.48)O3 target, which was20% larger than that used with Sample 1. The optimizedoxygen pressure for the boundary condition between the ox-idized mode and the metallic mode was 2 Pa (Ar:O2 = 1:3),as shown inFig. 4. Samples 2 and 3 adopted the same

Fig. 7. XRD spectrum of deposited PZT film by using Pb(Zr0.52, Ti0.48)O3

target on an unheated substrate.

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Fig. 8. XRD spectrum of deposited PZT film by using Pb1.2(Zr0.52, Ti0.48)O3

target, where the substrate was heated up at 470◦C.

sputtering condition. Sample 4 is a commercially-producedbulk PZT. Samples 2 and 3 differ in the lead compositionwithin the target. With Sample 2, it was 20% greater. Thelead composition was decreased with Sample 3 because lead

Fig. 9. Microscope images on 750 nm thickness of PZT thin film heated up at 470◦C.

Fig. 10. AFM image of a PZT cross section on substrate embedded in epoxy resin.

Fig. 11. SEM and AFM images 750 nm thick PZT thin films heated to 650◦C.

evaporates at temperatures over 450◦C. With Sample 3, theoxygen composition was increased to optimize the oxygenpressure conditions at 2 Pa (Ar:O2 = 1:3).

Crystalline structure analysis for deposited PZT thin filmswere performed by X-ray diffraction (XRD).Fig. 7 showsthe X-ray spectrum of a PZT thin film deposited on an un-heated glass substrate (Sample 1 inFig. 6). In this figure, thePZT thin film deposited without any heat treatment showsan amorphous structure. Conversely, when the substrate washeated to 470◦C (Sample 2 inFig. 6), PZT perovskite crys-talline structure was observed, as shown inFig. 8.

5. Improvement of surface quality for deposited PZTthin films

The piezoelectric function of PZT thin films depositedunder the sputtering conditions described inSections 2 and 3failed after the voltage was applied about 10 times. The sur-face of deposited PZT was observed by using microscope,

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Fig. 12. XRD spectrum of PZT film deposited onto a Pt/Ti/Invar substrateheated to 600◦C, using Pb1.2(Zr0.52, Ti0.48)O3 target.

and pinholes were found as shown inFig. 9. This typi-cal pinhole is about 1�m in diameter, with a surface areaof 0.03�m2.

Fig. 10shows a cross section through a PZT thin film de-posited using Pt electrodes (100 nm thickness) on an Invaralloy substrate embedded in epoxy resin. Because the PZTwas not deposited homogeneously, thin parts of PZT wereobserved inFig. 10. These thinner regions would lead to pin-holes. However, according to Ishihara et al.[7], the depositedfilms were smooth, dense and without pinholes when de-posited onto substrates heated above 600◦C in order to obtainthe perovskite phase without post-annealing.

In order to obtain a PZT thin film free from pinholes, thesubstrate was heated to above 600◦C in accordance withIshihara’s experiment.Fig. 11shows SEM and AFM imagesof PZT thin films produced in this way. By increasing thesubstrate temperature from 470 to above 600◦C, the pinholesobserved inFig. 9were eliminated.

Fig. 12 shows the X-ray spectrum from a PZT thin filmdeposited onto a Pt/Ti/Invar substrate, heated to 600◦C, usinga Pb1.2(Zr0.52, Ti0.48)O3 target. PZT(1 1 0) and PZT(1 1 1)crystalline orientation were observed by XRD analysis. TheX-ray spectrum intensity of PZT(1 1 0) is three times largerthan the PZT thin film heated to 470◦C, as shown inFig. 8.When PZT thin films are deposited on a substrate heated athigher temperature, more highly-orientated PZT thin film isobtained.

6. Piezoelectric property of deposited PZT thin film

In order to measure the piezoelectric property, the PZT dis-placement was measured with a Laser Doppler Vibrograph.The voltage is applied to piezoelectric PZT actuators acrossthe film thickness.Fig. 13shows a displacement of bulk PZTproduced by the Fuji Ceramics Corporation. A displacementof about 35 nm resulted when 55 V was applied. The piezo-electric propertyd33 is therefore 0.45 nm/V. In comparison,Fig. 14shows the displacement of PZT thin film actuator de-posited on Invar alloy heated to 470◦C. A voltage of 15 V

Fig. 13. Displacement of monomorph bulk PZT actuator as a function oftime(500 nm thickness).

Fig. 14. Displacement of deposited PZT actuator (500 nm thickness) as afunction of time (substrate temperature: 470◦C).

resulted in a displacement of about 10 nm. The piezoelectricpropertyd33 is 0.32 nm/V; 70% of the value from bulk PZT.Fig. 15shows the displacement of PZT thin film actuator de-posited on Invar alloy heated to at 600◦C. When 22 V wasapplied, the displacement was about 15 nm. The piezoelec-tric propertyd33 is 0.48 nm/V, very similar to the property ofbulk PZT.

Fig. 15. Displacement of deposited PZT actuator (500 nm thickness) as afunction of time (substrate temperature: 600◦C).

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Fig. 16. AFM image of deposited PZT thin film on heated and unheatedglass (Sample A: 470◦C, Sample B: unheated).

7. PZT grain size and piezoelectric property

Surfaces of PZT thin films, deposited onto substratesheated to various temperatures, were observed by AFMas shown inFig. 16. The temperature of Sample A wasraised to 470◦C, whilst Sample B was not heated. ThePZT deposited at ambient temperatures (Fig. 16B) is amor-phous and therefore shows no grain structure. When thesubstrate was heated (Fig. 16A), the grain size increasedand polycrystalline structure PZT was observed. All piezo-electric PZTs have a polycrystalline structure. However,a single crystal structure generally shows a much bet-

Fig. 17. PZT grain size as a function of substrate temperature.

Fig. 18. Piezoelectric propertyd33 as a function of PZT grain size.

ter piezoelectric property. Therefore, in order to obtain agreater grain size, a substrate should be heated during thedeposition.

Fig. 17shows the grain size of deposited PZT as a functionof substrate temperature. The grain size increases linearlywith substrate temperature.Fig. 18shows that the piezoelec-tric propertyd33 increases linearly with the increased PZTgrain size produced at higher substrate temperatures. Thepiezoelectric function was improved since the deviation froma specified perovskite crystal orientation decreases in the caseof larger grain size so that high intensities of (1 1 0) and (1 1 1)were obtained, as shown inFigs. 8 and 12. The higher sub-strate temperature increases the PZT grain size and improvesthe piezoelectric propertyd33.

8. Conclusions

PZT very thin films (500 nm thick) were deposited ontoa Pt/Ti/Invar alloy substrate using a high-temperature RFmagnetron sputtering system. The 20% lead-enriched target,Pb1.2(Zr0.52, Ti0.48)O3, was used in the high-temperaturesputtering procedure. Optimum conditions were identifiedin order to produce a high deposition speed, an sufficientlyoxidizing target, and a smooth surface. These involved anargon/oxygen total gas pressure of 2 Pa (Ar:O2 = 1:3), andan input power 100 W.

Finally, through the crystal structure analysis by XRD, andthe measurement of piezoelectric property by Laser DopplerVibrograph, the following effects of substrate temperaturewere found.

(1) Above 600◦C, pinholes on deposited PZT films disap-peared.

(2) At 600◦C, it was possible to fabricate a smooth PZT sur-face with a very small number of pinholes and roughnessless than 5 nm.

(3) PZT grain size increased linearly with substrate temper-ature at the time that the PZT was deposited.

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(4) Piezoelectric propertyd33 also increased linearly withthe increased PZT grain size produced at higher substratetemperatures.

(5) When the substrate temperature was 600◦C, the piezo-electric propertyd33 was almost same as commercially-produced bulk PZT.

However, lead oxide still remained in the deposited PZTthin films and this hindered the piezoelectric functions. Workis therefore needed to identify the sputtering conditions toeliminate lead oxide from refined PZT thin film.

Acknowledgments

The authors would like to thank Prof. Hata at KanazawaUniversity, for the discussion of the relationship betweenmetallic mode and oxidized mode for PZT deposition. I wouldalso like to acknowledge the help with the manuscript forDr. David Hayton in King Henry VIII School, Coventry, UK.This work was supported by bio venture grant from Min-istry of Education, Culture, Sports, Science and Technology,Japan.

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